WELDING ON CONTAINERS
Produced by:
Industrial Health and Safety Program,
Ontario Ministry of Labour
Alert #I12/0395
ISSN 1195-5228
HAZARD SUMMARY:
In two recent incidents, explosions during welding on fuel tanks
killed three workers. In the first, two workers welding a 150-
gallon diesel fuel tank were fatally injured. In the second, a
500-gallon gasoline tank exploded during welding, killing a
worker. These accidents would not have happened if proper
cleaning, purging and testing procedures had been followed.
After the first incident, it was discovered that the diesel fuel
in the tank was contaminated with gasoline. As little as two per
cent of gasoline in diesel fuel can create an explosive mixture
in a closed container, with the flashpoint (the temperature at
which a spark or other ignition source will cause an explosion)
falling below the temperature inside the container.
REQUIRED PRECAUTIONS:
For any hot work such as welding or cutting on a container that
may have contained flammable or combustible material, the
following minimum precautions must be taken:
- The container's internal layout must be determined to make
sure that fittings such as baffles will not interfere with
cleaning or purging.
- The container must be drained and cleaned using appropriate
methods.
- To determine whether draining and cleaning has made the
container safe, its interior must be tested with a combustible
gas detector both before hot work begins and periodically
during the work.
However, some containers cannot be drained and cleaned well
enough to make them safe. Such containers may be made safe by
purging and inerting with an inert gas, but only if these
precautions are taken:
- Recognized procedures and proper equipment must be used.
- The oxygen level inside the contianer must be monitored with
an oxygen analyser and maintained at essentially zero for the
duration of the work.
- Workers must be made aware of the limitations of the inerting
process.
NEVER ASSUME A CONTAINER IS CLEAN OR SAFE. MAKE SURE THAT IT IS
MADE SAFE AND THAT ITS SAFETY IS VERIFIED BY TESTING BEFORE ANY
HOT WORK BEGINS. NOT FOLLOWING THIS RULE IS LIKELY TO KILL YOU.
The Regulations for Industrial Establishments (R.R.O 1990, Reg.
851, s.78) require that where repairs or alterations are to be
made on a drum, tank, pipeline or other container, it must be
drained and cleaned or otherwise made free from any explosive,
flammable or harmful substance.
See Engineering Data Sheet 4-14, "Welding and Other Hot Work on
Containers" for more information (follows). For assistance,
contact your nearest Ministry of Labour office.
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ENGINEERING DATA SHEET 4-14
WELDING AND OTHER HOT WORK ON CONTAINERS
1. INTRODUCTION
Hot work on containers, including welding, grinding and
cutting, is one of the most dangerous operations in industry.
Containers that have or may have contained a fuel or any
other flammable, combustible or high-flashpoint material are
potentially lethal, as are containers that may have an
interior coating that becomes hazardous when heated.
Hydrogen gas generated by interior corrosion and explosive
dusts (for example, sugar, starch and coal dusts) may also
create hazardous situtations. In these cases, all that is
needed to cause a container to explode is the wrong
combination of temperature and oxygen, and an ingnition
source.
Many explosions and several recent fatalities have been
caused by the use of inappropriate procedures in cleaning,
purging and testing containers in preparation for hot work.
Workers have to be made aware that hot work on a container is
being performed on what is essentially an unexploded bomb and
that they must treat the process with suitable respect.
The basic rule that must be obeyed before performing hot work
on a container is:
NEVER ASSUME THAT A CONTAINER IS CLEAN OR SAFE. MAKE
SURE THAT IT IS MADE SAFE, AND THAT THIS IS VERIFIED BY
TESTING.
2. LEGISLATION
The Regulations for Industrial Establishments (R.R.O. 851/90,
made under the Occupational Health and Safety Act) address
repairs on containers that have contained flammable,
explosive or harmful substances. The regulations state
(s. 78):
". . . where repairs or alterations are to be made on a
drum, tank, pipeline or other container, the drum, tank,
pipeline or other container shall,
a) have internal pressures adjusted to atmospheric before
any fastening is removed;
b) be drained and cleaned or otherwise rendered free from
any explosive, flammable or harmful substance; and
c) not be refilled while there is any risk of vaporising
or igniting the substance that is being placed in the
drum, tank, pipeline or other container.
The Ontario Fire Code also addresses welding on containers
(5.17.3.3 (1)):
"Welding or cutting of metal containers shall not be
undertaken until the containers and compartments within
such containers have been cleaned of flammable and
combustible materials and checked with an explosion
meter."
3. CODES AND STANDARDS
A number of universally accepted codes, standards and
guidelines address the hazards of hot work, including welding
and cutting, on containers that have contained hazardous
substances. These publications explain in detail the safety
procedures to be followed before such work is carried out.
The following section is a brief summary of the information
in the standards referenced in section 5. Since it is an
outline only, workers intending to perform hot work should
review all relevant standards in detail.
4. MAKING A CONTAINER SAFE FOR HOT WORK
Listed below are the steps that must be taken before any hot
work is performed on a container that has held a flammable or
combustible liquid.
Note that any testing carried out using a combustible gas
detector, oxygen analyser or carbon monoxide analyser MUST be
carried out by a person who has been thoroughly trained in
the use of such instruments and who is aware of the
requirements and limitations of such testing.
All containers that may require entry of workers for any
purpose are considered to be confined spaces under the
Occupational Health and Safety Act and Regulations, and all
applicable requirements must be satisfied before entry. See
Engineering Data Sheet 7-01 "Entry Into Tanks and Other
Confined Spaces".
4.1 IDENTIFY CONTENTS AND CONTAINER CONSTRUCTION
The contents of the container should be identified
before any work is carried out. Remember, however, that
the contents may have been contaminated (e.g. with
gasoline in diesel fuel) and that container labels may
be misleading or incorrect. If it is not absolutely
clear what the contents are, it must be assumed that a
potentially explosive material may be present, and the
procedures outlined below must be followed.
It is also important to determine the internal
configuration of the container and to identify any
fittings (e.g., baffles or tubes) that may interfere
with draining, cleaning or purging. Any areas inside
the container that might trap explosive mixtures must be
identified. All internal fittings should be removed
where possible.
If there is any possibility that the container may have
contained any flammable, combustible or high flashpoint
material, or if the container may have an interior
lining or coating, precautions as set out in 4.2 through
4.5 MUST be taken.
4.2 DRAIN CONTENTS
The contents of the container must be drained
completely, either by opening the drain connection or by
other means if a drain connection is not available.
4.3 CLEAN CONTAINER
The container must be cleaned using the appropriate
method indicated in the referenced standards. Cleaning
methods include: water displacement (for water soluble
substances); hot chemicals, using an appropriate hot,
strong caustic solution such as sodium triphosphate
solution; low pressure steam in conjunction with hot
soda ash solution; or a combination of the above
methods. Before a chemical detergent is used, verify
with the manufacturer that it is appropriate for the
intended task.
4.4 TEST ATMOSPHERE
The interior of the container must be tested with a
properly calibrated combustible gas detector to make
sure that the cleaning operation has made the container
safe. (A combustible gas detector reads the percentage
of the lower explosive limit of the vapours present in
the container.) If the reading on the detector
indicates that flammable vapours are present, i.e. if
the needle moves, the cleaning operation must be
repeated. The testing procedure must be carried out
immediately before beginning hot work and periodically
during it.
4.5 INERT GAS PURGING
In some situations it may be impossible to remove all of
the potentially hazardous liquid or liquids inside a
container. Liquid residues may be trapped behind heavy
scale or rust or between a vessel's liner and shell and
so may not be easily detected. A second possibility is
that a vessel may have an interior coating or lining
that will produce flammable vapours when heated.
When examination after cleaning indicates that either of
these conditions exists or may exist, other precautions
must be taken before beginning hot work. These may
include, as a minimum, maintaining an inert atmosphere
inside the container with an inert gas.
Extreme care should be taken when using an inert gas
such as carbon dioxide or nitrogen to purge a container
and maintain a safe atmosphere inside it. The workers
must be familiar with the limitations of the inerting
method and the characteristics of the inert gas being
used. The oxygen content inside the container must be
maintained at essentially zero, i.e. as close to zero as
possible, during the entire period that the work is in
progress.
Performing inert gas purging without thorough knowledge
of the process or proper equipment is extremely
hazardous. Inadequate knowledge or equipment is likely
to create a false sense of security, leading to serious
injury or death.
The steps to be followed in the inert gas purging
procedure are as follows:
1. Close all container openings except fill and vent
connections.
2. Introduce the inert gas into the container through a
hose extending to near the bottom of the container.
All metal parts of the filling equipment must be
bonded to the container.
3. When inerting with carbon dioxide, low pressure must
be used to avoid the generation of static
electricity. Portable carbon dioxide extinguishers
must never be used for inerting because they may
discharge static electricty.
4. Maintain the flow of inerting gas throughout the hot
work.
5. The contents of the container must be tested before
beginning hot work and during it. Its oxygen
content can be measured by an oxygen analyser. When
carbon dioxide is used for inerting, oxygen can be
measured indirectly with a carbon dioxide indicator.
Each type of instrument must be suitable for its
purpose. An oxygen analyser must be capable of
reading zero per center oxygen, and a carbon dioxide
analyzer must be capable of reading 100 per cent
carbon dioxide. The accuracy of the instrument to
be used needs to be determined and taken into
account.
The concentration of flammable vapours present in the
container can be measured with a combustible gas
detector. Note, however, that combustible gas detectors
of the catalytic filament type may not give a true
reading when the oxygen content of the atmospher tested
is less than about 10 per cent. Without special
adaptation, detectors of this type cannot reliably
determine whether the inerting process has been
successful in eleiminating any explosvie mixture.
5. REFERENCES
a) ANSI/AWS F4.1 Recommended Safe Practices for the
Preparation for Welding and Cutting of
Containers That Have Held Hazardous
Substances.
b) NFPA 51B Standard for Fire Prevention in Use of
Cutting and Welding Processes.
c) NFPA 327 Standard Procedures for Cleaning or
Safeguarding Small Tanks and Containers.
d) IAPA Welding on Containers or in Confined
Spaces.
e) CSA W117.2 Safety in Welding, Cutting and Allied
Processes.
f) NFPA Fire Protection Handbook, 16th Edition.
Produced by:
Professional and Specialized
Services and the Occupational
Health and Safety Branch,
Ministry of Labour
ISSN 1201-1444